Thinking about an AU where Kaigaku didn't have a chance to impress Kokushibou when his group encountered him and he got cut down like the rest. All he managed was to mitigate the attack just enough to cling to life until the kakushi arrived and could get him emergency care.
Zenitsu gets a letter at the start of hashira training instead of at the end, and it's from Jii-chan, not about him, telling him the news.
Zenitsu had been slacking off on Tengen's training & getting scolded for it—yeah, yeah, yeah, he has to go on these inhumanly demanding runs! He knows! He knows and he's doing it! Don't you see his feet moving?!
After the letter, he sprints off without a word, racing faster than he ever has in his life.
He hates hates Hates HATES HATES it that he's stopped, made to bathe and get dressed before he's allowed to make for the Butterfly Estate to see Kaigaku.
He never got on well with his fellow disciple, but seeing him is hard. He's hooked up, bandaged & unconscious, to so many morphine drugged wires and weird contraptions of machines, barely breathing, that there's hardly space enough to sit and hold his hand.
Somebody takes Zenitsu's other hand, he almost can't think of who, but it must be Jii-chan. It has to be Jii-chan who tells him the Insect Hashira Shinobu & the demon Lady Tamayo have the skill to save Kaigaku, but they're at a critical juncture in developing a cure to turn Nezuko human and poisons to debilitate Muzan.
They do not have the time.
One demon slayer corps member versus humanity itself?
"I'm sorry, Zenitsu."
—No.
no no No no No no NO!!!!
He can't accept that. He WON'T accept that. There has to be another way!!!
…There is.
Zenitsu begs Lady Tamayo at her feet, weeping. Can't she change him? Demons heal so fast from any wound.
Tamayo is sympathetic but more than that she's disgusted. Furious. Demons are a blight on this world, and he would dare ask this of her?
She did it once.
She swore never again.
And there has only ever even been one successful lasting transformation from demon slayer to demon.
"You would condemn him to die in agony when as it is now he's in no pain?" she growls. "He could leave the world in peace, humanity intact. The chance of success the other way is infinitesimal. Even if he survives he would become the exact breed of monster that killed his comrades and left him in this state. Tell me, would he thank you?"
The question takes Zenitsu back to a night long ago, rain pounding on the windows, thunder rattling the glass, to two boys huddled close in the warm circle of an oil lamp's glow, growing stacks of playing cards between them.
Zenitsu had Kaigaku on the run, seconds from defeat. He'd rarely ever succeeded in anything where Kaigaku failed, and it was more precious still Jii-chan's older student deigned to spend their rare spare time together to play with Zenitsu at all.
He grew cautious protecting his projected victory, so near he could taste it. He knew this game well. If he bided his time, kept his cards close, he could eke it out to the end. Kaigaku took advantage of that caution. With nearly nothing, he went on the offensive. Zenitsu was left scrambling, unable to fend him off.
Kaigaku laughed in his face when he finally lost, flicked him hard in the forehead, called him all sorts of stupid.
"But how?!"
Kaigaku sneered. "Some chance is better than no chance, you crybaby coward. As long as you hold even one card, you can still win."
That's what Zenitsu has always admired about Kaigaku, as much as he's hated him, too. Work hard until the end. Never give up. Always find a way.
Would Kaigaku thank him if he lived to become a demon?
Zenitsu sniffs up his snot and wipes away his tears. This woman doesn't know Kaigaku at all. He looks up into Tamayo's slitted inhuman eyes and answers her:
